1. Field of the Invention
The present invention relates to lasers and optoelectronics and in particular to new compound optical sources of coherent radiation for generation of high power densities based on summation of the power of few separate laser components by an optical beam combiner. The laser system may be used for combination of beams irradiated by multiple diode lasers or a diode laser array employing beam combining components based on specially configured optical fiber and integrated optical waveguide system to pump optical fiber amplifiers or active crystals in miniature solid state lasers.
2. lnformation Disclosure Statement
For many industrial and medical uses, the key attribute of a laser system is the power density that can be delivered to a particular site or point. Another desirable aspect of power delivery systems is that the laser's output be delivered to points remote from the laser and without being limited by the lasers physical orientation. Employing inexpensive semiconductor diode lasers instead of standard lasers is preferable for many industrial applications, since these lasers offer simple alignment and can easily be substituted for service or repair, if required. For example, such diode lasers could replace conventional laser sources for welding, cutting and surface treatment or can be used for pumping miniature solid state lasers and optical fiber lasers or amplifiers. The power needed for these applications, however, exceeds the power available from single low-cost diode lasers and has therefore required the use of high cost, high output lasers. Because the targeted applications require power that exceeds that of a single diode laser, one can use an array of diode lasers whose individual beams should be combined by some optical system and then coupled into an active optical fiber or an active crystal.
Such diode lasers, typically rated around 1 W, exhibit output cross sections of 200 .mu.m by 1-2 .mu.m and divergences of 20.degree. and 40.degree. in the respective cross-sectional major and minor axes. The outputs of these individual diode lasers are combined in the present state of the art by capturing each diode laser's output with a standard round optical fiber. The diameter of the core of the individual fibers is chosen such that it will encompass or cover a diode laser's largest output dimension, e.g., in the case of a 200 .mu.m by 1-2 .mu.m output pattern the capturing fiber's core diameter will be 200 .mu.m; with a typical thin cladding this would be at least a 220 .mu.m clad fiber. Typically cylindrical lenses are affixed to the diode lasers or to the end of the individual receiving fibers to reduce beam divergence. Groups of these individual fibers are then combined by being bound together, and packaged in a launch connector.
All optical systems have an inherent phase volume stated by Streibel's invariant, which for circular cross sections is d.sup.2 .eta. sin.THETA., where d is the beam diameter, .THETA. is the angle of the beam transverse divergence and .eta. is refractive index of the medium. Brightness of the beam is proportional to thin invariant and can be increased only by increasing the beam power. The beam power density, however, is inversely proportional to the beam cross section, so an increase in the cross section yields a significant decrease in power density. Since the phase volume cannot be decreased by an optical system, the goal of power delivery system should be the improve in brightness at the output of the system and the increase in beam power by minimizing loss in the system, particularly at system junctions, or the increase in power density by decreasing the beam cross section.
Present systems significantly decrease brightness of a diode laser's output at the coupling between each individual diode laser and optical fiber capturing the diode laser's beam. This is due to the difference in cross-sectional area dictated by the cross-sectional shape and the difference in numerical apertures of the laser and fiber. The diode laser has rectangular-shaped output and different numerical apertures in the directions of both principal axis of the rectangular cross section. The optical fiber used as the receiver has a circular cross-section, with a diameter approximately equal to the longer rectangular axis of the laser's output and matches numerical aperture of the laser only in this direction. Another loss of power and decrease of brightness occurs at the junction between the bundled, individual fibers coming from individual diode lasers and the single standard circular cross-sectional fiber into which the collected output is sent.
FIG. 1 shows the prior art, where a high loss interface 5 occurs between diode lasers 2 and combiner subsystem 6 of round fibers 4, and a second loss interface 8 occurs where the combiner subsystem's output 11 is sent into a receiving port 29 of delivery subsystem 12 having a diameter similar to that of the bundled output. At each of these interfaces the power density decreases due to losses and the increase in cross-sectional area of the receiving fibers. Fiber 13 carries the total captured output of the diode laser array and is considered the deliverable output of the system.
Current art is focused on the problem of minimizing the power loss at the high loss-prone junction between an optical device such as a diode laser and a single round optical fiber. The solutions used in current art focus on the creation of micro-lenses at the end of a round optical fiber to focus the light beam into the fiber, or the insertion of a lens between the source and the fiber to achieve the same purpose. Neither reduces the coupling losses at that junction. U.S. Pat. No. 5,256,851 describes the use of an asymmetric hyperbolic micro lenses on a single mode optical fiber to enhance the coupling efficiency of a diode and a fiber by matching the ellipsoidal ratios of the laser diode and the lens. This improves upon earlier microlenses described in U.S. Pat. Nos. 5,011,254 and 4,932,989. U.S. Pat. No. 5,127,068 teaches how to couple the light from an array of laser diodes into a plurality of bundled optical fibers using a cylindrical microlens made from a small diameter multimode optical fiber. U.S. Pat. No. 4,723,257 describes a laser diode pumped solid state laser comprising an optical fiber transmitting pumping radiation. These approaches are not practical, however, when the output of the diode laser has axial ratios larger than 10:1, as is typically encountered in high-power multimodal diode lasers. U.S. Pat. Nos. 4,818,062 and 4,688,884 describe an optical fiber having improved capability to receive light energy from a diode laser. The fiber has a tapered input end squashed into an elongated cross section to match both the diode laser numerical apertures and its cross section. The squashing of the fiber end can not, however, provide the ratio of the fiber cross dimensions as large as 1:20 required for effective matching to the diode laser. Even in this case the tapered section only serves as yet another optical means to input diode radiation into the final circularly symmetric fiber cross section. Moreover, tapering of the fiber from its elongated end to its round end changes rectangular configuration of the input laser beam which may be undesirable for certain applications. For example, typical diode laser array has irradiates 12 beams of rectangular cross section oriented along its horizontal plane of p-n junction. To recombine these beams in such a way that long axes of all the beams are oriented in the vertical direction one need flexible means to transmit a plurality of the beams without changing of brightness and shape of each of the beams. Therefore, for many practical applications, it is desirable to have means of efficient coupling of the light from diode lasers into optical fibers while preserving brightness and rectangular shape of the light beam irradiated by diode laser over whole the fiber length. To solve this problem, a special beam combiner systems must be designed.
The effective beam combiner is also a key component of many compound laser systems based on constructive superposition of beams generated by a number of individual sources of coherent radiation such as diode or fiber lasers. For example, the output radiation power of semiconductor lasers can be increased, retaining the spatial coherence, by the use of an array of strip structures formed on the same substrate and coupled either directly by the tunnel effect or indirectly by intermediate stripe waveguides. Although the output power can be increased considerably in systems of this kind, the problem of cooling the substrate remains and it has not yet been solved satisfactorily.
In the case of a fiber laser, the output power of the fiber lasers is governed by the pump radiation power coupled into the active fiber material, i.e. it is also limited by the power and spatial coherence of the semiconductor laser usually employed as the pump source.
One technique to achieve high-power laser operation is to spatially separate subgroups of lasers and then coherently combine their outputs.
Corcoran C. J. and Rediker R. H. (Appl. Phys. Lett. v.59, 759 (1991)) have demonstrated a system of five diode gain elements, i.e. semiconductor diode lasers having one facet antireflection-coated, operated as a coherent ensemble by use of an external cavity controlled by a spatial filter or a hologram. The gain elements spatially separated and fiber coupled into the cavity. Collection of the fibers with microlenses on their facets, bulk lenses and spatial filters formed the means for phase-coherent superposing of radiation form the separate lasers in the system. The disadvantages of this system are the bulky design, i.e. the using of a separate optical elements such as lenses, filters, holograms. As a consequence, this reduces the efficiency of summation of output radiation and results in increasing of the threshold of the laser system generation. Moreover, in this system it is difficult to avoid parasitic feedback due to undesirable reflections. When a system of this kind is used, additional devices are needed to concentrate the radiation in a single-mode waveguide channel.